Psoriasis is a distressing and often debilitating disease affecting up to 2% of the US population. The long- term goal of this project is to understand the pathogenesis of psoriasis. Substantial data indicate that immunosuppressants block psoriatic epidermal hyperplasia by inhibiting immunologic activation. However, the mechanism by which immunologic activation triggers epidermal hyperplasia remains unknown. Keratinocytes overexpress multiple ErbB1 ligands in psoriasis, including amphiregulin (AR), TGF-a, and HB- EGF. Of these, AR plays a predominant role in driving autocrine proliferation of cultured normal human keratinocytes (NHK). Transgenic mice overexpressing AR in skin reproducibly develop a severe but steroid- responsive psoriasiform skin rash and an inflammatory arthritis. Based on these findings, we hypothesize (1) that AR has one or more in vivo activities not shared by other EGF-like growth factors;(2) that AR provokes epidermal hyperplasia in psoriasis via evocation of cutaneous inflammation;and (3) that AR release from keratinocytes is stimulated by immune / inflammatory cells, thereby creating a vicious cycle. To test these hypotheses, we propose the following specific aims: 1. To determine the specificity of the psoriatic epidermal hyperplastic response for AR among ErbB ligands in vivo. This will be accomplished (a) by determining the profile of ErbB ligand shedding in PP organ cultures with and without TNF-a treatment, and (b) by treating mice bearing PP xenografts with neutralizing antibodies against AR, HB-EGF, TGF-a, EREG, and ErbB1, followed by analysis of signal transduction and epidermal regenerative maturation. 2. To assess the ability of activated immunocytes to specifically induce the synthesis and release of AR in KG. This will be accomplished by treating NHK and organ cultures of normal skin with cells or conditioned media derived from Strep extract-activated autologous PBMCs, followed by assessment of AR and other ErbB ligand mRNAs by QRT-PCR, and cell-bound and shed ligands by ELISA. 3. To develop a viable mouse model of AR-driven psoriasiform hyperplasia. This will be accomplished by generating K5 promoter-floxed EGFP-AR transgenic mice and mating them to K5-Cre-ERT2 mice. 4. To utilize the model developed in Aim 3 to elucidate the role of AR in cutaneous inflammation. This will be accomplished by (a) treating KLEAR mice with CsA and topical steroid before and during induction of AR expression, and (b) grafting KLEAR skin onto highly immunocompromised SCID mice to eliminate systemic immunity. If successful, this research will have important public health consequences not only in psoriasis but also in a wide variety of dermatoses characterized by epidermal hyperplasia.